The invention relates to a profiled element for rotary drilling equipment and, in particular, to a profiled portion of a rod of a rotary drill rod string.
In the field of prospecting for and production from oil deposits, use is made of rotary drill rod strings made up of rods and possibly other tubular elements which are joined together end to end as required for the drilling.
Such strings of rods may, in particular, make it possible to make deflected drillings, that is to say drillings whose incidence with respect to the vertical or the direction in terms of azimuth can be varied during drilling.
In the case of deflected drillings with a large offset including horizontal or practically horizontal portions, the friction couples due to the rotation of the drill lining may reach extremely high values during drilling. The friction couples may jeopardize the equipment used or the objectives of the drilling. Furthermore it is often difficult to bring up the debris produced by the drilling, given the sedimentation of the debris that occurs in the borehole, particularly in the steeply inclined part of the borehole. This results in poor cleaning of the hole and an increase both in the coefficients of friction of the rods of the string of rods inside the borehole and of the areas of contact between the rods and the walls of the hole.
In order to reduce the coefficient of friction and the area of contact between the string of rods and the walls of the borehole and to improve the cleaning of the borehole and the removal of debris in the drilling fluid, there has been proposed, in patent application FR-97/03207, a drill rod comprising at least one bearing region which has a central bearing part and two end portions, one on each side of the central bearing region and comprising, on their external surface, at least one groove arranged in a helix and the transverse cross section of which has an undercut part. The bearing region of the drill rod, which has a diameter greater than the diameter of the end portions, comes into contact with the wall of the borehole and provides a certain reduction in the friction between the drill rod and the wall of the borehole. The end parts which comprise hydraulic profiles allow flow of drilling fluid to be activated and allow debris attached to the wall of the borehole to be detached.
The bearing parts of the drill rod generally comprise grooves which allow the drilling fluid to pass between the bearing parts and the wall of the borehole. The drilling fluid flowing through these grooves in an axial direction has practically no effect in reducing the friction between the bearing parts of the drill rod and the wall of the hole. The effect that the bearing parts of the drill rod has on reducing the friction is therefore limited.
Bearings are known in which the friction can be reduced down to very low levels, by the hydrodynamic effect of a fluid circulated between the rubbing surfaces of the bearing. In the case of the profiled drill rod string elements known from the prior art and as described, for example, in FR 97 03207, such hydrodynamic effects of the drilling fluid between the bearing surfaces of the drill rod and the wall of the hole have not been able to be obtained. In general, numerous elements for drilling equipment are known, including drill rods with a cylindrical overall shape, that is to say exhibiting a cylindrical external envelope surface, which comprise projecting parts and recessed parts on their external surface which are arranged in helixes with, as their axis, the axis of rotation of the drilling equipment. Such profiled shapes including projecting parts and recessed parts in helixes make it possible, in particular, to improve the flow of the drilling fluid in the annular space delimited between the equipment and the borehole. However, these profiled elements, the transverse cross section of which is constant along the longitudinal axial direction of the drilling equipment and the diameter of which is smaller than the diameter of the borehole, provide no solution regarding the production of low-friction bearings for guiding the equipment or the drill rod.
What is more, when tension is exerted on drilling equipment, such as a string of rods, to raise it back to the surface, blockages may occur on account of the drilling equipment becoming jammed by debris or roughnesses projecting from the wall of the borehole. It may be very difficult if not impossible to unblock and therefore raise the drilling equipment. The recessed or projecting helix-shaped parts present on the exterior surface of certain drilling equipment such as drill rods do not generally provide any solution to this problem or, on the contrary, increase the risk of jamming.
The object of the invention is therefore to provide a profiled element for rotary drilling equipment, exhibiting a shape which is overall of revolution and an axis directed along the axis of rotation of drilling, and parts which project and parts which are recessed in radial directions on its external surface, in arrangements which are roughly in the shape of helixes with, as their axis, the axis of rotation of the drilling equipment, this profiled element making it possible, in particular, to reduce the friction between the drilling equipment and a wall of a borehole and to limit the risks of the drilling equipment jamming when raising the equipment back up inside the borehole.
To this end, over at least part of the length of the profiled element in the axial direction, at least one of the geometric and dimensional characteristics of the recessed parts and of the radial projecting parts varies in the axial direction of the element.